Why it happens I don’t know and I’m not even sure the “experts” know either but besides screwing with the ability to remember things, dementia messes with the person in many other ways. They can become more paranoid or they may go from being mean to lovey dovey and vice versa. Some change from being neat to being super over the top neat or conversely – very sloppy.
My mother went from being neat and clean and nice (but never warm and fuzzy unless it was with the grandkids) to being so terribly messy, unkempt, and eventually she went through a phase where she wanted to hug and kiss everyone. Oddly, that lovey phase just was weird because for my entire life she was never like that and it just made me uncomfortable. Sad but true.
I’ve seen where the one with dementia would gain weight not because they were that hungry but because they couldn’t remember eating and would eat again thinking they hadn’t eaten or that someone had refused to feed them. Mom didn’t have that issue. Fortunately something in her told her to stop cooking and dad took over that task (although I’m not sure how well they were eating as he was getting sicker too) but even with that she began to exhibit odd behavior with meals.
If she ate something and got the hiccups or a bit of gas she would refuse to eat that food again! Funny, she could remember that something gave her an upset stomach but the rest of her memory was misfiring. Or maybe it was that she was saying all of those things upset her stomach and she limited her diet to the few things she could remember. Either way, her diet dwindled down to bottles upon bottles of Pepsi Cola (that was the only cola she would touch) and peppermint candies. Dad did manage to get other food in her but their trashcan would be filled to the brim with empty cola bottles and the floor around the indoor wastebasket was littered with the peppermint wrappers.
When they had to go into nursing homes and we cleaned out their old house there were so many bottles and wrappers in there I had to resort to using a snow shovel to scoop them all up! Another behavior change was that she would get confused about temperatures; either over dressing in layers when it wasn’t that cool or she would fiddle with the thermostat and turn the furnace up to just this side of Hades even in the summer! That last habit proved to be very dangerous later on.
As a caregiver – be it living with your loved one or having to stop by often to check up on them – your work will be cut out for you and my best advice is to get help! If your loved one doesn’t threaten to clobber you and the person/s you hire to come in to help out (like mom did to me and an agency I enlisted to help – that was one ugly scene!) and you can afford it – do that! Enlist the help of other family members if you can and they will actually help you. When all else fails, prioritize! You can’t and you won’t be able to win them all; work with what you can as well as with what is most important.
Remember – if it comes down to your loved one eating the same thing every day (as long as it’s not unhealthy for them) versus an outright battle, let them eat the same thing! You might even be able to sneak in some other food if you can disguise it enough; think how some parents have to try and get veggies into their children. With bathing if they refuse to get into the tub or shower then go with sponge bathing. If all else fails gather up your strength for a mighty once or twice a week bath or shower fight. You will both be less stressed if you attempt this once or twice a week instead of every day. There’s grubby and there’s GRUBBY! You will learn to differentiate.
That reminds me of something. Mom hated taking a shower in the nursing home to the point that they had to give her a mild sedative (how I hate the memory of that) to get her in the tub but one day an aide decided to take her and put her in the whirlpool. She liked that so much that they no longer had to give her any medication to take a bath. So maybe switching from bath to shower or the other way around could work for you. All you can do is try and realize you aren’t a failure at being a caregiver if something doesn’t work out…you just found something that didn’t on the way towards what will!